According to the state of the art the processes of directed crystallization are carried out in apparatuses of different kinds operating on different principles. One of them is equipped with a localized heating system for generating a homogeneous temperature field remaining in standing position and means for low-speed forwarding of the crystals within the temperature field. Different solutions of this kind have become known, such as the Bridgman-Stockberger method or the Czochralski method etc. There are also known methods of crystallization, wherein a lengthy piece of material is arranged in a fixed position and a heating zone surrounding the piece is translated along the longitudinal axis of the piece of the material (e.g. zone melting or refining process, floating zone method or travelling heater method). The common negative features of the processes and methods listed up above are the following:
--remarkable differences of temperature can be experienced within the same piece of material, the differences causing elastic tensions and lattice imperfections (defects) in the solid crystalline lattice and in the melted zone the flow of the material, resulting in undesired and uncontrollable changes of concentration of selected components, PA1 --the crystal or the heating zone is translated along the longitudinal axis by means of a motor which is, when operating, the source of vibration and microacceleration, which are especially detrimental in space conditions. PA1 --in radial directions the temperature differences can be minimized, PA1 --in longitudinal direction the temperature differences can be controlled and lowered to the required extent, PA1 --the crystallization process is carried out without relative movement of different pads of the apparatus. PA1 --the power consumption is remarkably high and it is as higher the temperature gradient desired to be generated, PA1 --the gas filling out the internal space is in convective flow which cannot be controlled in any way, PA1 --the furnace offers the possibility of producing only one working piece (crystal).
The features mentioned above impose a very significant limitation to the possibility of producing crystals with a reduced number of lattice imperfections.
The disadvantages mentioned above can be avoided in some of the multizone apparatuses for crystallization, wherein such control is realized whereby
The specification to U.S. Pat. No 4,423,515 discloses a multizone furnace capable of generating temperature differences having a large slope gradient and excluding the undesired radial heat flow within the working space of the furnace. The furnace consists of annular elements which define isolating and heating layers. The heating elements of the heating layers are arranged in recesses of the isolating rings. The outer ring of the heating layer is made of a material having very good heat conductivity (especially aluminum) whereby effective heat removal from the outer surface of the furnace is ensured. The outlets of the heating elements are turned by 90.degree. with respect to one another. Means for sensing the temperature may be arranged within holes prepared in the isolating elements.
The heat removal from the outer surface is intended to make the steady heat introduction through the heating elements necessary. Thereby it is guaranteed that, if required, the temperature can be changed very quickly and that heat flow is generated in the working space only in an axial direction. The heat removal from the outside of the furnace is ensured by contacting the surfaces with an air stream, cooling water or other cooling medium.
Heating is controlled by a respective control unit operating on the basis of a loaded program stored in a memory unit or modified after loading.
The specification to U.S. Pat. No. 4,734,127 discloses a method of purifying aluminium by carrying out fractional crystallization in the atmosphere of a protective gas filling out the internal space of a cylindric furnace. The contents of the internal space are heated by a higher number of heating elements arranged one over another. The mentioned heating elements are divided by at least one cooling zone wherein the crystals are growing. The crystals are removed from the inner surface by a compacting scraper which forwards them to the lower regions of the furnace and compacts them. According to the teaching of this patent the power consumption of some of the heating elements surrounding the compacted crystals can be reduced.
In this specification, the drawings include figures showing two kinds of furnace, the first having one heating zone and the second having three such zones. The specification discloses also the embodiment of the compacting scraper equipped with a heated mandrel and a motor for lowering, and further a doubled embodiment of the furnace.
The solutions analyzed above are also loaded with some disadvantages and especially the following:
The cosmic investigations involve specific problems. The crystallization processes to be carried out in the cosmic space are intended to make use of conditions where only microgravitation may be present and it is intended to reduce microgravitation as much as possible. Any mechanical motion results in deterioration of the gravitation conditions and if equipped with a motor or other mechanical means operating during crystallization, the respective apparatuses cannot be prepared with microgravitation lower than 10.sup.-5 g (g means the gravitational acceleration). Hence, in the case of apparatuses for carrying out directed crystallization processes in astronautical research, it is especially important to exclude mechanical motions. When growing crystals in the cosmic space conditions it is extremely important to ensure that the melting crucible remain motionless.
The generic problem of the cosmic space investigations is that the energy consumption of any device applied should be very low. The intensity of the energy dissipation has to be limited also to a very low extent, e.g. to 50 W for units. Otherwise, the temperature control in the inner space of the cosmic vehicle may become difficult. This impose very strict limitations to the equipment used for energy transformation, e.g. to the different kinds of the DC to AC converters which should show efficiency as high as at least 90%. A further requirement is that no substance may be deliberated by any apparatus and especially by a crystallization apparatus which can cause corrosion, explosion or is capable of shielding the telecommunication means. Another requirement is that with regard to the high costs of preparing and operating the different kinds of equipment applied in the cosmic space vehicles the apparatus should be capable of producing as high of a number of different materials as possible. The operation should be realizable during the full flight time in an autonomous way with high reliability.